MicroRNA is a recently characterized class of small, non-coding, RNA that repress mRNA translation. Work over the past several years has revealed important roles for microRNA in a vast array of developmental and disease-related processes. Within the developing mammalian central nervous system, results from dicer null mice support a role for microRNAs in neuronal morphogenesis and neuronal survival. However, relatively little is known about how neuronal activity regulates microRNA expression patterns in the mature nervous system and, importantly, whether microRNA regulate neuronal plasticity and cell viability. Based on recent work by a number of investigators, and on the preliminary data reported here, we propose that microRNA plays a key role in activity-dependent structural plasticity in the mature nervous system. To test this hypothesis we have assembled a novel set of genetically modified mouse models, and an array of genetic and functional screening assays. In Aim 1, we propose to utilize the Solexa deep sequence method to examine activity-dependent expression of non-coding RNA in the hippocampus. We will also examine the contribution of transcriptional networks that underlie activity-dependent neuronal plasticity and perform a series of experiments to identify functionally relevant microRNA targets. In Aim 2 we propose to determine the contribution of microRNA to adult neuronal structural plasticity and neuroprotection. To this end, we will employ an inducible form of Cre-recombinase to disrupt Dicer expression. The effects on both physiological and pathophysiological levels of neuronal activity will be examined. In Aim 3, we propose to determine the role of the microRNA-132 locus in activity-induced structural remodeling in vivo. A combination of knockout and tet-inducible microRNA mouse strains will be used to test this question. The data generated here should provide a wealth of new insights regarding how neuronal activity sculpts microRNA expression patterns, and, in turn, how these changes affect key aspects of neuronal plasticity and pathology.